It has long been known that synthetic esters can be used both as a base oil and as an additive in lubricants. By comparison with the less expensive, but environmentally less safe mineral oils, synthetic esters were mostly used as base oils in cases where the viscosity/temperature behavior was expected to meet stringent demands and a low pour point was required, as in aircraft turbine oils. Recently, esters based on oleochemicals have been acquiring increasing significance because they combine high performance levels with biodegradability. Esters based on oleochemicals are generally divided into 5 groups, namely: monoesters, glycerol esters, dicarboxylic acid esters, polyol esters and complex esters. For technological and economic reasons, the dicarboxylic acid esters are derived above all from adipic acid, trimethyl adipic acid, sebacic acid, azelaic acid, dodecanedioic acid and brassylic acid, C.sub.6-12 dicarboxylic acid derivatives in particular acquiring increasing significance because they show very good viscosity/temperature behavior. In the field of lubricating oils, the viscosity/temperature ratio is generally characterized by the socalled viscosity index (VI) which is determined at 40.degree. and 100.degree. C. in accordance with DIN 51562 and calculated in accordance with DIN ISO 2909. High VI values indicate that minor variations in viscosity are observed at various temperatures. Most of the dicarboxylic acid esters mentioned above have high VI values. Thus, linear dicarboxylic acid esters of adipic acid in particular, with VI values of 200 and higher, show very little dependence of viscosity on temperature. However, the disadvantage of the above-mentioned dicarboxylic acid esters of adipic acid is their low viscosity so that they are not suitable for all fields of application.
Complex esters made up of polybasic carboxylic acids and polyols and monocarboxylic acids show higher viscosities than adipic acid esters. Unfortunately, however, these complex esters do not exhibit such good viscosity/temperature behavior and, hence, have lower VI values than the dicarboxylic acid esters of adipic acid.
Although both the adipic acid esters and the complex esters show acceptable low-temperature behavior insofar as the pour point, i.e. the temperature obtained by adding 3.degree. C. to the temperature read off, at which the sample no longer flows after cooling, is of the order of -30.degree. C., it is of advantage for certain applications for the base oil to have an even lower pour point.
The problem addressed by the present invention was to provide base oils which would show improved low temperature behavior (pour point) and which, in addition, would have higher viscosities than the known adipic acid esters. In addition, these base oils would exhibit very good viscosity/temperature behavior and would have VI values above 200.